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<div class="section" id="module-mmap">
<span id="mmap-memory-mapped-file-support"></span><h1>18.9. <a class="reference internal" href="#module-mmap" title="mmap: Interface to memory-mapped files for Unix and Windows."><code class="xref py py-mod docutils literal"><span class="pre">mmap</span></code></a> — Memory-mapped file support<a class="headerlink" href="#module-mmap" title="Permalink to this headline">¶</a></h1>
<hr class="docutils" />
<p>Memory-mapped file objects behave like both <a class="reference internal" href="functions.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal"><span class="pre">bytearray</span></code></a> and like
<a class="reference internal" href="../glossary.html#term-file-object"><span class="xref std std-term">file objects</span></a>. You can use mmap objects in most places
where <a class="reference internal" href="functions.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal"><span class="pre">bytearray</span></code></a> are expected; for example, you can use the <a class="reference internal" href="re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal"><span class="pre">re</span></code></a>
module to search through a memory-mapped file. You can also change a single
byte by doing <code class="docutils literal"><span class="pre">obj[index]</span> <span class="pre">=</span> <span class="pre">97</span></code>, or change a subsequence by assigning to a
slice: <code class="docutils literal"><span class="pre">obj[i1:i2]</span> <span class="pre">=</span> <span class="pre">b'...'</span></code>. You can also read and write data starting at
the current file position, and <code class="xref py py-meth docutils literal"><span class="pre">seek()</span></code> through the file to different positions.</p>
<p>A memory-mapped file is created by the <a class="reference internal" href="#mmap.mmap" title="mmap.mmap"><code class="xref py py-class docutils literal"><span class="pre">mmap</span></code></a> constructor, which is
different on Unix and on Windows. In either case you must provide a file
descriptor for a file opened for update. If you wish to map an existing Python
file object, use its <code class="xref py py-meth docutils literal"><span class="pre">fileno()</span></code> method to obtain the correct value for the
<em>fileno</em> parameter. Otherwise, you can open the file using the
<a class="reference internal" href="os.html#os.open" title="os.open"><code class="xref py py-func docutils literal"><span class="pre">os.open()</span></code></a> function, which returns a file descriptor directly (the file
still needs to be closed when done).</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">If you want to create a memory-mapping for a writable, buffered file, you
should <a class="reference internal" href="io.html#io.IOBase.flush" title="io.IOBase.flush"><code class="xref py py-func docutils literal"><span class="pre">flush()</span></code></a> the file first. This is necessary to ensure
that local modifications to the buffers are actually available to the
mapping.</p>
</div>
<p>For both the Unix and Windows versions of the constructor, <em>access</em> may be
specified as an optional keyword parameter. <em>access</em> accepts one of three
values: <code class="xref py py-const docutils literal"><span class="pre">ACCESS_READ</span></code>, <code class="xref py py-const docutils literal"><span class="pre">ACCESS_WRITE</span></code>, or <code class="xref py py-const docutils literal"><span class="pre">ACCESS_COPY</span></code>
to specify read-only, write-through or copy-on-write memory respectively.
<em>access</em> can be used on both Unix and Windows. If <em>access</em> is not specified,
Windows mmap returns a write-through mapping. The initial memory values for
all three access types are taken from the specified file. Assignment to an
<code class="xref py py-const docutils literal"><span class="pre">ACCESS_READ</span></code> memory map raises a <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal"><span class="pre">TypeError</span></code></a> exception.
Assignment to an <code class="xref py py-const docutils literal"><span class="pre">ACCESS_WRITE</span></code> memory map affects both memory and the
underlying file. Assignment to an <code class="xref py py-const docutils literal"><span class="pre">ACCESS_COPY</span></code> memory map affects
memory but does not update the underlying file.</p>
<p>To map anonymous memory, -1 should be passed as the fileno along with the length.</p>
<dl class="class">
<dt id="mmap.mmap">
<em class="property">class </em><code class="descclassname">mmap.</code><code class="descname">mmap</code><span class="sig-paren">(</span><em>fileno</em>, <em>length</em>, <em>tagname=None</em>, <em>access=ACCESS_DEFAULT</em><span class="optional">[</span>, <em>offset</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap" title="Permalink to this definition">¶</a></dt>
<dd><p><strong>(Windows version)</strong> Maps <em>length</em> bytes from the file specified by the
file handle <em>fileno</em>, and creates a mmap object. If <em>length</em> is larger
than the current size of the file, the file is extended to contain <em>length</em>
bytes. If <em>length</em> is <code class="docutils literal"><span class="pre">0</span></code>, the maximum length of the map is the current
size of the file, except that if the file is empty Windows raises an
exception (you cannot create an empty mapping on Windows).</p>
<p><em>tagname</em>, if specified and not <code class="docutils literal"><span class="pre">None</span></code>, is a string giving a tag name for
the mapping. Windows allows you to have many different mappings against
the same file. If you specify the name of an existing tag, that tag is
opened, otherwise a new tag of this name is created. If this parameter is
omitted or <code class="docutils literal"><span class="pre">None</span></code>, the mapping is created without a name. Avoiding the
use of the tag parameter will assist in keeping your code portable between
Unix and Windows.</p>
<p><em>offset</em> may be specified as a non-negative integer offset. mmap references
will be relative to the offset from the beginning of the file. <em>offset</em>
defaults to 0. <em>offset</em> must be a multiple of the ALLOCATIONGRANULARITY.</p>
</dd></dl>
<dl class="class">
<dt>
<em class="property">class </em><code class="descclassname">mmap.</code><code class="descname">mmap</code><span class="sig-paren">(</span><em>fileno</em>, <em>length</em>, <em>flags=MAP_SHARED</em>, <em>prot=PROT_WRITE|PROT_READ</em>, <em>access=ACCESS_DEFAULT</em><span class="optional">[</span>, <em>offset</em><span class="optional">]</span><span class="sig-paren">)</span></dt>
<dd><p><strong>(Unix version)</strong> Maps <em>length</em> bytes from the file specified by the file
descriptor <em>fileno</em>, and returns a mmap object. If <em>length</em> is <code class="docutils literal"><span class="pre">0</span></code>, the
maximum length of the map will be the current size of the file when
<a class="reference internal" href="#mmap.mmap" title="mmap.mmap"><code class="xref py py-class docutils literal"><span class="pre">mmap</span></code></a> is called.</p>
<p><em>flags</em> specifies the nature of the mapping. <code class="xref py py-const docutils literal"><span class="pre">MAP_PRIVATE</span></code> creates a
private copy-on-write mapping, so changes to the contents of the mmap
object will be private to this process, and <code class="xref py py-const docutils literal"><span class="pre">MAP_SHARED</span></code> creates a
mapping that’s shared with all other processes mapping the same areas of
the file. The default value is <code class="xref py py-const docutils literal"><span class="pre">MAP_SHARED</span></code>.</p>
<p><em>prot</em>, if specified, gives the desired memory protection; the two most
useful values are <code class="xref py py-const docutils literal"><span class="pre">PROT_READ</span></code> and <code class="xref py py-const docutils literal"><span class="pre">PROT_WRITE</span></code>, to specify
that the pages may be read or written. <em>prot</em> defaults to
<code class="xref py py-const docutils literal"><span class="pre">PROT_READ</span> <span class="pre">|</span> <span class="pre">PROT_WRITE</span></code>.</p>
<p><em>access</em> may be specified in lieu of <em>flags</em> and <em>prot</em> as an optional
keyword parameter. It is an error to specify both <em>flags</em>, <em>prot</em> and
<em>access</em>. See the description of <em>access</em> above for information on how to
use this parameter.</p>
<p><em>offset</em> may be specified as a non-negative integer offset. mmap references
will be relative to the offset from the beginning of the file. <em>offset</em>
defaults to 0. <em>offset</em> must be a multiple of the PAGESIZE or
ALLOCATIONGRANULARITY.</p>
<p>To ensure validity of the created memory mapping the file specified
by the descriptor <em>fileno</em> is internally automatically synchronized
with physical backing store on Mac OS X and OpenVMS.</p>
<p>This example shows a simple way of using <a class="reference internal" href="#mmap.mmap" title="mmap.mmap"><code class="xref py py-class docutils literal"><span class="pre">mmap</span></code></a>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">mmap</span>
<span class="c1"># write a simple example file</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s2">"hello.txt"</span><span class="p">,</span> <span class="s2">"wb"</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
<span class="n">f</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">b</span><span class="s2">"Hello Python!</span><span class="se">\n</span><span class="s2">"</span><span class="p">)</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s2">"hello.txt"</span><span class="p">,</span> <span class="s2">"r+b"</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
<span class="c1"># memory-map the file, size 0 means whole file</span>
<span class="n">mm</span> <span class="o">=</span> <span class="n">mmap</span><span class="o">.</span><span class="n">mmap</span><span class="p">(</span><span class="n">f</span><span class="o">.</span><span class="n">fileno</span><span class="p">(),</span> <span class="mi">0</span><span class="p">)</span>
<span class="c1"># read content via standard file methods</span>
<span class="nb">print</span><span class="p">(</span><span class="n">mm</span><span class="o">.</span><span class="n">readline</span><span class="p">())</span> <span class="c1"># prints b"Hello Python!\n"</span>
<span class="c1"># read content via slice notation</span>
<span class="nb">print</span><span class="p">(</span><span class="n">mm</span><span class="p">[:</span><span class="mi">5</span><span class="p">])</span> <span class="c1"># prints b"Hello"</span>
<span class="c1"># update content using slice notation;</span>
<span class="c1"># note that new content must have same size</span>
<span class="n">mm</span><span class="p">[</span><span class="mi">6</span><span class="p">:]</span> <span class="o">=</span> <span class="n">b</span><span class="s2">" world!</span><span class="se">\n</span><span class="s2">"</span>
<span class="c1"># ... and read again using standard file methods</span>
<span class="n">mm</span><span class="o">.</span><span class="n">seek</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">mm</span><span class="o">.</span><span class="n">readline</span><span class="p">())</span> <span class="c1"># prints b"Hello world!\n"</span>
<span class="c1"># close the map</span>
<span class="n">mm</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p><a class="reference internal" href="#mmap.mmap" title="mmap.mmap"><code class="xref py py-class docutils literal"><span class="pre">mmap</span></code></a> can also be used as a context manager in a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal"><span class="pre">with</span></code></a>
statement.:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">mmap</span>
<span class="k">with</span> <span class="n">mmap</span><span class="o">.</span><span class="n">mmap</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="mi">13</span><span class="p">)</span> <span class="k">as</span> <span class="n">mm</span><span class="p">:</span>
<span class="n">mm</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">b</span><span class="s2">"Hello world!"</span><span class="p">)</span>
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.2: </span>Context manager support.</p>
</div>
<p>The next example demonstrates how to create an anonymous map and exchange
data between the parent and child processes:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">mmap</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="n">mm</span> <span class="o">=</span> <span class="n">mmap</span><span class="o">.</span><span class="n">mmap</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="mi">13</span><span class="p">)</span>
<span class="n">mm</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">b</span><span class="s2">"Hello world!"</span><span class="p">)</span>
<span class="n">pid</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">fork</span><span class="p">()</span>
<span class="k">if</span> <span class="n">pid</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span> <span class="c1"># In a child process</span>
<span class="n">mm</span><span class="o">.</span><span class="n">seek</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">mm</span><span class="o">.</span><span class="n">readline</span><span class="p">())</span>
<span class="n">mm</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>Memory-mapped file objects support the following methods:</p>
<dl class="method">
<dt id="mmap.mmap.close">
<code class="descname">close</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.close" title="Permalink to this definition">¶</a></dt>
<dd><p>Closes the mmap. Subsequent calls to other methods of the object will
result in a ValueError exception being raised. This will not close
the open file.</p>
</dd></dl>
<dl class="attribute">
<dt id="mmap.mmap.closed">
<code class="descname">closed</code><a class="headerlink" href="#mmap.mmap.closed" title="Permalink to this definition">¶</a></dt>
<dd><p><code class="docutils literal"><span class="pre">True</span></code> if the file is closed.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.2.</span></p>
</div>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.find">
<code class="descname">find</code><span class="sig-paren">(</span><em>sub</em><span class="optional">[</span>, <em>start</em><span class="optional">[</span>, <em>end</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.find" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns the lowest index in the object where the subsequence <em>sub</em> is
found, such that <em>sub</em> is contained in the range [<em>start</em>, <em>end</em>].
Optional arguments <em>start</em> and <em>end</em> are interpreted as in slice notation.
Returns <code class="docutils literal"><span class="pre">-1</span></code> on failure.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.5: </span>Writable <a class="reference internal" href="../glossary.html#term-bytes-like-object"><span class="xref std std-term">bytes-like object</span></a> is now accepted.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.flush">
<code class="descname">flush</code><span class="sig-paren">(</span><span class="optional">[</span><em>offset</em><span class="optional">[</span>, <em>size</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.flush" title="Permalink to this definition">¶</a></dt>
<dd><p>Flushes changes made to the in-memory copy of a file back to disk. Without
use of this call there is no guarantee that changes are written back before
the object is destroyed. If <em>offset</em> and <em>size</em> are specified, only
changes to the given range of bytes will be flushed to disk; otherwise, the
whole extent of the mapping is flushed.</p>
<p><strong>(Windows version)</strong> A nonzero value returned indicates success; zero
indicates failure.</p>
<p><strong>(Unix version)</strong> A zero value is returned to indicate success. An
exception is raised when the call failed.</p>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.move">
<code class="descname">move</code><span class="sig-paren">(</span><em>dest</em>, <em>src</em>, <em>count</em><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.move" title="Permalink to this definition">¶</a></dt>
<dd><p>Copy the <em>count</em> bytes starting at offset <em>src</em> to the destination index
<em>dest</em>. If the mmap was created with <code class="xref py py-const docutils literal"><span class="pre">ACCESS_READ</span></code>, then calls to
move will raise a <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal"><span class="pre">TypeError</span></code></a> exception.</p>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.read">
<code class="descname">read</code><span class="sig-paren">(</span><span class="optional">[</span><em>n</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.read" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a <a class="reference internal" href="functions.html#bytes" title="bytes"><code class="xref py py-class docutils literal"><span class="pre">bytes</span></code></a> containing up to <em>n</em> bytes starting from the
current file position. If the argument is omitted, <code class="docutils literal"><span class="pre">None</span></code> or negative,
return all bytes from the current file position to the end of the
mapping. The file position is updated to point after the bytes that were
returned.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.3: </span>Argument can be omitted or <code class="docutils literal"><span class="pre">None</span></code>.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.read_byte">
<code class="descname">read_byte</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.read_byte" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns a byte at the current file position as an integer, and advances
the file position by 1.</p>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.readline">
<code class="descname">readline</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.readline" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns a single line, starting at the current file position and up to the
next newline.</p>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.resize">
<code class="descname">resize</code><span class="sig-paren">(</span><em>newsize</em><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.resize" title="Permalink to this definition">¶</a></dt>
<dd><p>Resizes the map and the underlying file, if any. If the mmap was created
with <code class="xref py py-const docutils literal"><span class="pre">ACCESS_READ</span></code> or <code class="xref py py-const docutils literal"><span class="pre">ACCESS_COPY</span></code>, resizing the map will
raise a <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal"><span class="pre">TypeError</span></code></a> exception.</p>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.rfind">
<code class="descname">rfind</code><span class="sig-paren">(</span><em>sub</em><span class="optional">[</span>, <em>start</em><span class="optional">[</span>, <em>end</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.rfind" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns the highest index in the object where the subsequence <em>sub</em> is
found, such that <em>sub</em> is contained in the range [<em>start</em>, <em>end</em>].
Optional arguments <em>start</em> and <em>end</em> are interpreted as in slice notation.
Returns <code class="docutils literal"><span class="pre">-1</span></code> on failure.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.5: </span>Writable <a class="reference internal" href="../glossary.html#term-bytes-like-object"><span class="xref std std-term">bytes-like object</span></a> is now accepted.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.seek">
<code class="descname">seek</code><span class="sig-paren">(</span><em>pos</em><span class="optional">[</span>, <em>whence</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.seek" title="Permalink to this definition">¶</a></dt>
<dd><p>Set the file’s current position. <em>whence</em> argument is optional and
defaults to <code class="docutils literal"><span class="pre">os.SEEK_SET</span></code> or <code class="docutils literal"><span class="pre">0</span></code> (absolute file positioning); other
values are <code class="docutils literal"><span class="pre">os.SEEK_CUR</span></code> or <code class="docutils literal"><span class="pre">1</span></code> (seek relative to the current
position) and <code class="docutils literal"><span class="pre">os.SEEK_END</span></code> or <code class="docutils literal"><span class="pre">2</span></code> (seek relative to the file’s end).</p>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.size">
<code class="descname">size</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.size" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the length of the file, which can be larger than the size of the
memory-mapped area.</p>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.tell">
<code class="descname">tell</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.tell" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns the current position of the file pointer.</p>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.write">
<code class="descname">write</code><span class="sig-paren">(</span><em>bytes</em><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.write" title="Permalink to this definition">¶</a></dt>
<dd><p>Write the bytes in <em>bytes</em> into memory at the current position of the
file pointer; the file position is updated to point after the bytes that
were written. If the mmap was created with <code class="xref py py-const docutils literal"><span class="pre">ACCESS_READ</span></code>, then
writing to it will raise a <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal"><span class="pre">TypeError</span></code></a> exception.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.5: </span>Writable <a class="reference internal" href="../glossary.html#term-bytes-like-object"><span class="xref std std-term">bytes-like object</span></a> is now accepted.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="mmap.mmap.write_byte">
<code class="descname">write_byte</code><span class="sig-paren">(</span><em>byte</em><span class="sig-paren">)</span><a class="headerlink" href="#mmap.mmap.write_byte" title="Permalink to this definition">¶</a></dt>
<dd><p>Write the integer <em>byte</em> into memory at the current
position of the file pointer; the file position is advanced by <code class="docutils literal"><span class="pre">1</span></code>. If
the mmap was created with <code class="xref py py-const docutils literal"><span class="pre">ACCESS_READ</span></code>, then writing to it will
raise a <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal"><span class="pre">TypeError</span></code></a> exception.</p>
</dd></dl>
</dd></dl>
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